UNITED NATIONS (AP) — U.S. Ambassador Samantha Power shrugged off President Vladimir Putin’s claim that Russia might support a U.N. resolution authorizing military force against Syria, saying Moscow has held the Security Council “hostage” on the Syria crisis and Washington does not expect that to change.

Power lashed into Russia in her most extensive comments yet on the Syria crisis since starting her job as ambassador to the United Nations last month. Her criticism came as President Barack Obama was in Russia for an economic summit of the G-20 nations.

“Russia continues to hold the Council hostage and shirks its international responsibilities,” Power said.

Putin said in an interview with The Associated Press this week that Russia “doesn’t exclude” backing military action against Syria if it is proven that President Bashar Assad’s regime was behind an Aug. 21 chemical weapons attack near Damascus. Russia has said it is not convinced by the evidence the United States has cited.

The U.S. Mission held briefings Thursday with most U.N. member countries to present the Obama administration’s assessment of the attack. Power said the facts presented “overwhelmingly point” to the conclusion that Assad’s regime was responsible.

Power said Russia has consistently used its veto power to block Security Council action against Syria since the civil war broke out 2 ½ years ago, or blocked the council from issuing statements by consensus. She said she sees no reason to believe that pattern will change.

“I have seen Putin’s comments,” Power told reporters. “There is nothing in the pattern of our interactions with our colleagues in the Security Council, with our Russian colleagues, that would give us any reason to be optimistic.”

“And indeed, we have seen nothing in President Putin’s comments that suggest that there is an available path forward at the Security Council,” she added.

Russia’s U.N. Mission declined to comment on Power’s remarks.

Power accused Moscow of exploiting the Security Council’s voting system, which lets five major nations hold veto power — Russia, the United States, China, France and Britain.

“The system has protected the prerogatives of Russia, the patron of a regime that would brazenly stage the world’s largest chemical weapons attack in a quarter century, when chemical weapons inspectors sent by the United Nations were just across town,” Power said.

The U.N. inspectors left Syria over the weekend after gathering evidence for their investigation into the Ghouta attack.

Power said nothing about China, which has more quietly joined Russia in vetoing several Syria resolutions.

Power, a former senior adviser to Obama on human rights, was a strong advocate of U.S. intervention in Libya when Moammar Gadhafi’s forces were gaining the upper hand against rebels. She advocated use of U.S. air power, as did then U.N. Ambassador Susan Rice, who is now Obama’s s national security adviser.